1. Field of the Invention
The present invention relates to a projection lens suitable for use in plate-making or duplication which chiefly takes images substantially at real size.
2. Description of the Prior Art
Among plate-making lenses generally used in the prior art, a lens assembly having a relatively long focal length, about 400 mm or more, is mostly composed of four lenses which are arranged as shown in FIG. 1. The covering angle thereof is in the order of from 40.degree. to 50.degree. and the F-number thereof is in the order of 9 to 16. In such lens assemblies, it is difficult to correct the curvature of field. At the intermediate angle of view, the sagittal image surface in the curvature of field becomes convex to negative. At the margin, contrarily, the sagittal image surface changes to be convex toward positive to a large extent. As the angle of view is near 50.degree., the meridional image surface in the curvature of field changes to negative at the marginal portion to a great extent. Although it is recently desired to improve the plate-making lens assemblies with respect to quality of image, it is difficult to obtain a plate-making lens of reduced size and providing highly improved images because of longer focal length.